With the progress in scientific technologies as well as the modularization and miniaturization of camera lens, it is now possible to produce digital cameras having a very small volume, and most of currently available mobile phones are provided with the function of a digital camera. There are various types of auto-focus driving structures for the conventional miniature lens. Among others, the voice coil motor (VCM) is the currently most widely employed auto-focus driving structure. This is because the VCM has the advantages of small volume, low power consumption, accurately actuated displacement, and cost-effective, and is therefore very suitable for short-distance driving in miniature lens auto-focusing.
FIG. 1 is an exploded perspective view of a conventional auto-focus module, which includes an outer frame 10 serving as a housing, a bottom cover 11, a lens holder 12 provided with internal threads 15, four magnets 14 separately mounted to four inner wall surfaces of the outer frame 10, a lens 16 screwed into the lens holder 12, a winding 17 fitted around the lens holder 12, and an upper and a lower spring plate 18, 19 for supporting the lens holder 12 at a center of the outer frame 10, so that the lens holder 12 is axially forward and rearward movable in the outer frame 10 along a center axis thereof. When the winding 17 is excited by supplying electric current thereto, a magnetic field is produced for the winding 17 and the magnets 14 to magnetically repel or attract one another, so that the lens holder 12 and the lens 16 screwed thereto are brought to axially move forward or rearward to focus automatically.
FIG. 2 is a perspective view of an assembly of the outer frame 10, the magnets 14 and the winding 17 shown in FIG. 1. As can be clearly seen in FIG. 2, the magnets 14 do not cover four inner corners 20 of the outer frame 10. Therefore, the electromagnetic field produced by the excited winding 17 does not magnetically repel or attract the magnets 14 at the four inner corners 20, causing waste of the electromagnetic field and reduced driving efficiency. This means more power must be consumed to drive the miniaturized auto-focus module.
Therefore, the conventional auto-focus module requires further improvement to avoid waste of the electromagnetic field. For this purpose, it is tried by the inventor to develop a magnet holding structure of auto-focus module, in which four inverted L-shaped plates are separately provided at four inner corners of the outer frame to downward extend from a top of the outer frame and correspond to the spaces existing between any two adjacent magnets, so as to enhance the magnetic efficiency between the magnets and the winding.